1. Field of the Invention
The present invention relates to a polycarbosilastyrene copolymer, a method for producing the same and a process for producing a shaped or molded silicon carbide article from the same.
More particularly, the present invention relates to a polycarbosilastyrene copolymer which is easily shaped to a desired form and converted to a sintered silicon carbide article, a method for producing the same from a corresponding polysilastyrene copolymer, and a process for producing a shaped silicon carbide article for example, silicon carbide filaments or films, from the same.
2. Description of the Related Art
Various methods have been attempted for producing shaped, sintered silicon carbide articles, for example, silicon carbide filaments or films. In particular, methods for producing silicon carbide filaments which include (1) chemical deposition, (2) chemical conversion of precursory carbon filaments, and (3) conversion of precursory polymer filaments. Among the above-mentioned methods, the chemical deposition method and the precursory polymer filament-conversion method are usually employed for producing commercial silicon carbide filaments.
The chemical deposition method includes a method in which carbon filaments or tungsten filaments are brought into contact with silicon tetrachloride gas, hydrogen carbide gas, and hydrogen gas so as to allow the resultant silicon carbide to deposit on the carbon or tungsten filaments.
In another method, which is close to the chemical deposition method, precursory carbon filaments are brought into contact with silicon which has been produced by the heat decomposition of silicon tetrachloride, to cause the precursory carbon filaments to be converted to silicon carbide filaments.
In the above-mentioned two methods, the deposition of silicon carbide and the conversion of carbon to silicon carbide are effected only at an elevated temperature, for example, 1200.degree. to 1600.degree. C. Therefore the cost of the methods is very high and the resultant silicon carbide filaments have a large thickness, which means that the resultant filaments can be used only in limited fields and only in a restricted usage.
In the precursory polymer filament-conversion method, precursory filaments are produced from an organic silicon polymer, that is, polycarbosilane, and then converted to non-fusible polycarbosilane filaments by cross-linking, and finally to silicone carbide filaments by sintering or baking.
This precursory polymer filament-conversion method was invented for the first time by Professor Yajima et al of the Metal Material Research Institute of Tohoku University, and has been disclosed in Japanese examined patent publication Nos. 57-26527, 58-38534, 58-38535, and 59-33681, and Japanese unexamined patent publication Nos. 56-9209 and 56-74126
Also, Japanese unexamined patent publication No. 58-213026 discloses a polysilane polymer cross-linked with an organic metal compound and a process for producing the cross-linked polysilane polymer.
The above-mentioned method for producing silicon carbide filaments from the polycarbosilane polymer is capable of producing silicon carbide filaments wherein individual filaments each have a diameter of 10 .mu.m to 50 .mu.m. However, this method consists of a number of steps, a long time is needed to complete the method and, therefore, the work is difficult. For example, the method includes a first step in which compounds having Si-C bonds, compounds having Si-C and Si-H bonds, compounds having Si-halogen atom bonds, compounds having Si-N bonds, compounds having Si-OR wherein R represents an organic radical, compounds having Si-OH bonds, compounds having Si-Si bonds, and/or compounds having Si-OS bonds are polymerized at a temperature of from 300.degree. C. to 1200.degree. C. to produce an organic silicone polymer containing a silicon-carbon backbone structure; a second step in which the organic silicone polymer material is subjected to a treatment effective for decreasing the content of low molecular weight compounds in the polymer material and the resultant produced is subjected to an organic solvent extraction procedure or to an aging procedure in vacuum or in an atmosphere of air, CO.sub.2 gas or an inert gas at a temperature of from 50.degree. C. to 700.degree. C. to provide an organic silicone polymer; a third step in which the organic silicone polymer is subjected to a spinning procedure to provide organic silicone polymer filaments; a fourth step in which the filaments are heated in an oxidative atmosphere at a relatively low temperature of 50.degree. C. to 350.degree. C. for a time of several minutes to 3 hours; a fifth step in which the low temperature heated filaments are preliminarily heated in vacuum or in an atmosphere of an inert gas, CO gas or hydrogen gas at a temperature of 350.degree. C. to 800.degree. C.; and a sixth step in which the preliminarily heated filaments are sintered under vacuum or in an atmosphere of an inert gas, CO gas or hydrogen gas at a high temperature of 800.degree. C. to 1200.degree. C., to provide silicon carbide (SiC) filaments.
The above-mentioned method for producing the silicon carbide filaments from the polysilane polymer includes 6 steps. Japanese examined patent publication No. 50-38534 discloses a modified silicon carbide-producing method in which the fourth low temperature heating step in an oxidative atmosphere at a temperature of 50.degree. C. to 300.degree. C. for several minutes to 3 hours is omitted. In this modified method, in the second step, it is necessary to sufficiently reduce the content of the low molecular weight compounds in the organic silicon polymer material, and this of necessity, prolongs the time for the second step and requires greater care. Also, in the fifth step, it becomes necessary for the polymer filaments to be heated to a temperature of from 350.degree. C. to 800.degree. C. under vacuum to sufficiently remove the remaining low molecular weight compounds by means of distillation or evaporation. In this fifth step, if the removal of the low molecular weight compounds is unsatisfactorily carried out, the resultant filaments in the fifth step are melt-bonded to each other, and therefore, the resultant filaments are useless as high strength filaments or as reinforcing filaments.
In order to eliminate the above-mentioned disadvantages, Professor Robert West provided a new method in which polysilastyrene was used as an organic silicone polymer and polysilastyrene filaments were easily converted to silicon carbide filaments without applying the heating step in an oxidative atmosphere. This method is disclosed in Japanese unexamined patent publication No. 58-215426.
In this method, the polysilastyrene polymer is cross-linked and is made non-fusible by radiating electron beam or electron spectra having a wave length shorter than that of visible rays.
The important difference between the polysilastyrene-using method and the conventional polycarbosilane-using methods is as follows.
In the method in which a polycarbosilane polymer is used as a spinning material, for example, a method now industrially utilized, a dimethylpolysilane is produced by polymerizing dichlorodimethylsilane in a solvent consisting of a hydrocarbon, for example, toluene or xylene, in the presence of a sodium catalyst. The resultant dimethylpolysilane has a melting point which is very close to a heat decomposing point thereof, and therefore, cannot be directly melt-spun to provide polycarbosilane filaments. Accordingly, it is necessary to convert the dimethylpolysilane to a soluble, fusible polycarbosilane by heat-treating it in an autoclave at a temperature of from 400.degree. C. to 800.degree. C., and then to melt-spin the resultant polycarbosilane to provide filaments.
In the silicon carbide filament-producing method from a polysilastyrene polymer, dichlorodimethylsilane and dichloromethylphenylsilane are polymerized in a toluene or xylene solvent in the presence of a sodium catalyst to provide polysilastyrene (dimethyl-methylphenyl polysilane). This polysilastyrene polymer is soluble and fusible, and therefore, can be easily shaped into filaments or films by means of a melt-shaping or dry-shaping method. The polysilastyrene polymer has a melting point or softening point of from 80.degree. C. to 130.degree. C., and is readily converted to filaments having a very small thickness of 10 .mu.m to 30 .mu.m.
However, the polysilastyrene filaments are disadvantageous in that, when cross-linked and made non-fusible by ultra-violet ray radiation, the filaments are frequently fuse-bonded to each other and are easily shrunk. Also, the amount of shrinkage is very large, such that the length of the shrunk filaments is 1/3 to 1/2 the original length of the non-shrunk filaments. Therefore, the filament product cannot be maintained in the original form of a non-shrunk product.
Accordingly, in order to obtain silicon carbide filaments or films having a high mechanical strength from polysilastyrene polymer material, the ultra-violet ray radiation must be applied to the polysilastyrene filaments or films under a very appropriate intensity of tension or in a decreased intensity of radiation of ultra-violet rays within a vacuum chamber for a long time. Also, in the next sintering step, the temperature must be elevated from the ambient temperature to the desired sintering temperature, for example, 1200.degree. C., over a long time of from 20 to 200 hours. Accordingly, this method of conversion of polysilastyrene to silicon carbide is not always advantageous. That is, it is difficult to industrially produce a shaped silicon carbide article having a high mechanical strength by the method wherein a polysilastyrene polymer material is directly shaped to a desired form, for example, a filament or film.
It is known that sintered silicon carbide articles can be produced by adding a bonding agent (sintering assistant agent) consisting of an organic silicon compound, or boron and carbon powder, or an inorganic oxide selected from magnesium oxide, yttrium oxide or aluminum oxide, to the silicon carbide article.
The methods wherein the organic silicon compound is used as a bonding agent include a method disclosed in Japanese unexamined patent publication No. 52-103407, wherein an organic silicon compound having Si-C bonds, Si-Si bonds, and/or Si-OR bonds, in which R represents an organic radical, or a polycarbosilane polymer is used, and another method disclosed in Japanese unexamined patent publication No. 58-215426 in which a polysilastyrene polymer is used. However, the use of an organic silicon compound is disadvantageous in that it is difficult to produce a sintered article from the organic silicon compound monomer alone, and in that when the bonding agent consists of the organic silicon compound alone, the resultant sintered article exhibits an unsatisfactory mechanical strength.
That is, in a conventional method of producing a sintered silicon carbide article by using an organic silicon compound, although a shaped article can be easily produced, the shaped article is undesirably melted and deformed in a heating step for making the shaped article non-fusible Accordingly, in order to make the shaped article non-fusible while maintaining the shaped article in the desired shape, the heating procedure should be carried out at a relatively low temperature of, for example, 50.degree. C. to 450.degree. C., for a long time of 20 to 500 hours
Also, it is very difficult to directly convert a shaped article made from a polycarbosilane polymer material or polysilastyrene polymer material alone to a shaped silicon carbide article by sintering or baking. Accordingly, the polycarbosilane and polysilastyrene polymer materials are utilized only as a bonding agent for silicon carbide. That is, in the conventional method, the polycarbosilane or polysilastyrene polymer material is mixed as a bonding agent with silicon carbide powder, the mixture is shaped (molded) to a desired form, and the shaped (molded) article is heated to modify the article to a non-fusible state and then sintered to provide a shaped silicon carbide article But, even in this conventional method, the polycarbosilane and polysilastyrene is frequently decomposed in the sintering procedure. If the sintering reaction is carried out at a high temperature-elevating rate, the shaped (molded) article is sometimes partially melted and deformed, or foams are generated in the shaped (molded) article. Therefore, the resultant sintered shaped (molded) article exhibits an unsatisfactory quality and property.